Specific Aims: Drs. Jonsson and Linial propose collaborative experiments to analyze the RNA: protein interactions leading to genomic RNA encapsidation for two different classes of viruses, hantaviruses and retroviruses. The Jonsson lab brings to this proposal expertise in hantavirology and biochemistry and chemistry of RNA-protein interactions. The Lineal lab provides the expertise in retrovirology and molecular genetic analysis of RNA-protein interactions using yeast. For retroviruses, we have chosen the avian or alpharetroviruses, Rous sarcoma virus (RSV). Work on the packaging signal of these viruses has pin pointed a small, contiguous RNA sequence that interacts specifically with residues in the nucleocapsid (NC ) domain of the viral structural protein precursor Gag. For the hantaviruses, we will use the Hantaan virus (HTNV). A putative packaging signal has been identified in the S- segment of the HTNV to interact with the nucleoprotein (N). The two specific aims of this pilot project are: Aim 1). To identify the domain/amino acids in the HTNV N protein responsible for interaction with viral genomic and anti-genomic RNA using the yeast three-hybrid assay (YTHA), and Aim 2). To characterize the chemistry of the interaction of the RSV NC with its packaging signal, psi.